Hemodynamic and hormonal changes were studied in 13 sheep whose entire circulation was maintained by a biventricular bypass system with a pair of pusher-plate type blood pumps. Ventricular fibrillation was induced and pneumatically driven pumps placed outside the body kept the animals alive for 2-48 days (average: 11 days). Pump output and aortic pressure stayed within physiological ranges, but central venous pressure was elevated from 5 to 10-13 ㎜Hg after surgery. Serum levels of antidiuretic hormone, cortisol and insulin were stabilized soon after surgery. Adrenaline level and renin activity did not change significantly. For unknown reasons, the noradrenaline level showed a temporary increase after a week. Atrial natriuretic peptide (ANP) increased significantly 5 days after surgery. There were significant drops in triiodothyronine (T(3)) and thyroxine (T(4)) levels after surgery with gradual recovery afterward to 40% and 50% of the control levels, respectively. These results suggested that the biventricular bypass system maintained hemodynamic conditions similar to those maintained by the natural heart, although ANP, T(3) and T(4) values were changed.